In high-speed offset printing, an endless paper web up to 72" wide at a temperature of about 80.degree. F. is fed through an offset press where it is printed on both sides with a thermoplastic ink and then passed into a dryer. The dryer heats the web to a range 260.degree. F.-380.degree. F. to dry the printing by evaporating the solvents in the ink. The ink is still at the 260.degree. F. to 380.degree. F. temperature when the web leaves the dryer, and because the ink is thermoplastic it remains capable of being deposited on surfaces it may contact until it is "set" by cooling. The depositing of ink is referred to as ink picking and smearing, and it destroys the readability of the printed product. Therefore the web is passed through a chill roll stand having a series of chill rolls where the ink is set by cooling to about 90.degree. F. or less. Offset press speeds of 2000-2500 feet per minute (over 28 miles of printed paper web every hour) are common.
In order to successfully operate an offset press at these high speeds there are a number of interrelated operating parameters that must be met to minimize or avoid problems. Examples of these parameters and problems are: web tension, web weaving (lateral shifting of the web from side to side), moisture content, static electricity, air film between the web and the chill roll, solvent condensation on the chill roll and in the web, and condensate streaking (condensed solvent on the chill roll and in the web redissolving some of the ink back into liquid form which creates ink streaks on the web making the printed product unusable). Frequently the measures taken to adjust one operating parameter to solve one problem exacerbate another problem and therefore the setting of operating parameters has always involved compromising desired standards to a significant extent.
For example, because of the incredibly high speed of the press, the traveling web must be held at high tension to minimize web weaving and lateral shifting of the web from side-to-side and web breakage. In order to maintain proper high tension, the web must be dry and must remain dry; but a dry web has known serious shortcominqs. First, a dry web is more brittle than a moist web and subject to greater likelihood of breakage resulting in press downtime. For example, if the web breaks, in one minute there will be about one-half mile of paper on the pressroom floor. Second, the combination of the dry web and the high-speed travel thereof results in generation of static electricity which makes the web difficult to handle because of clinging. It has long been known that proper moisture will virtually eliminate both breakage and static electricity problems. But experience has taught that moistening of the web is to be avoided because moisture causes even greater problems. For example, moistening of the web causes the paper fibers to expand and such expansion allows the web to stretch resulting in loss of web tension and/or baggy edges to the web. The loss of web tension is highly detrimental as it will allow the web to weave as previously mentioned, and weaving can result in the web breaking unless the weaving can be immediately controlled or stopped. If the weaving cannot be controlled, the press must be shut down and a lengthy, complex start-up procedure followed to restart and recenter the web. Attempts have been made to moisten the web by misting or fogging with water or causing moist air to condense on the chill roll as disclosed in U.S. Pat. No. 2,157,388, issued May 9, 1939 to C. J. MacArthur, but the results have not been satisfactory because of the inability to precisely control the amount of moisture added to the web. Therefore, while moisturizing the web to eliminate brittleness and generation of static electricity is highly desirable, it is avoided to prevent expansion of fibers and consequent loss of web tension.
Another problem is that of efficiently transferring heat from the web to the chill roll. As previously explained, the function of the chill roll is to cool and set the ink after the hot web leaves the dryer. As the speed of the offset press increases, the amount of time a given area of the traveling web is in contact with the chill roll decreases with a consequent decrease in heat transfer time and cooling efficiency. Because of the known problems that arise as a result of moisturizing the web, it is the practice to circulate cooling water internally of the chill roll to avoid directly moisturizing the web. When a specific peripheral area on the chill roll comes into interfacial contact with an opposing area of the web, the cold metal surface of the chill roll will warm up to a higher interface temperature which will be identified as TS and the web surface which faces the metal will in turn cool down to the same temperature TS. The object is to make the temperature TS cool enough to set the ink sufficiently so that it can tolerate the subsequent physical contact of handling rolls downstream of the first chill roll without ink picking and smearing of the print.
The specific value of TS will depend upon the various properties of the chill roll, especially the temperature thereof, the temperature of the web entering the chill roll, and the thermal properties of both the web and the ink thereon such as specific heat, density, and thermal conductivity. The product of these three thermal properties will be termed the index of contact temperature preservation. The higher this index is, the closer the value of TS will be to the oncoming temperature of the material with the high index. On the side of the web which faces the first chill roll (web/chill roll side), this index is quite high because the chill roll is metal which has an inherently high thermal conductivity and density. Therefore there usually is no problem in creating an interfacial temperature TS low enough at the web/chill roll side to set the ink. Unfortunately, the duration of the interfacial contact is so short in a high-speed press that the low cooling temperature created on the web/chill roll side does not have time to penetrate through the web to aid in cooling the ink on the opposite surface of the web thereby resulting in a partially cooled web. Thus the web can leave the chill roll with the ink on the web/chill roll side set while the ink on the opposite side is not set.
There have been many strategies adopted to minimize the partial cooling problem. Obviously the press can be run at a slower speed to increase contact time but this is unacceptable because it increases the cost of production. Improving the intimacy of contact between the web and chill roll helps. When the web wraps around the chill roll, a film of air is trapped between the peripheral surface of the chill roll and the web and this air film acts as an insulator to reduce heat transfer. Improving the intimacy of the web/chill roll side contact by using a pressure roll as taught in U.S. Pat. No. 3,442,211, issued May 6, 1969 to F. W. Beacham, or using an air nozzle on the opposite side of the web, as taught in U.S. Pat. No. 4,369,584, issued Jan. 25, 1983 to Robert A. Daane, is known to help heat transfer but not enough to provide complete cooling.
One obvious solution to the problem would be to water cool both the pressure roll made of metal and the chill roll which is in nip pressure contact with it. Then both rolls will have an outer surface of metal having a high index of contact temperature preservation. However, metals do not deform very much under pressure and therefore it is very difficult to maintain a uniform nip pressure with two mating metal rolls. It is possible to make rolls with sufficient accuracy so that the remaining dimensional inaccuracies can be absorbed by compression of the paper web in the nip but thermal expansion and contraction must also be accounted for. In all known present chill roll designs, the outer chill roll shell is firmly supported only by roll heads at the ends of the roll and the shell is free to thermally expand or contract as dictated by the temperature profile across the web width. If there is a relatively lower temperature of the outer chill roll shell existing locally at some region across the width, that part of the shell will contract to a smaller diameter than will exist in other regions of the shell. This smaller diameter will lead to less nip pressure and still less heating of that part of the shell by the hot paper web. Thus temperature nonuniformity of the chill roll outer shell is self-amplified, leading to serious nonuniform web pressure and nonuniform cooling across the web width. Therefore the use of a metal pressure roll having a high index of contact temperature preservation is not practical.
To avoid the problems arising from using a metal pressure roll, U.S. Pat. No. 3,442,211 teaches the use of an elastomeric pressure roll. While this provides more uniform pressure and avoids distorting the ink, the index of contact temperature preservation for an elastomeric pressure roll is very low and not effective for cooling the opposite side of the web which it contacts. In operation the elastomeric surface immediately becomes hot and does not cool and set the ink. Instead the pressure roll will pick and smear the ink to destroy the readability of the print. U.S. Pat. No. 3,442,211 recognizes the ink picking problem and to avoid it teaches that the pressure roll be covered with an ink resistant material such as a silicone compound or a synthetic plastic such as "Teflon". However, such coatings also have a low index of contact temperature preservation and will not efficiently cool and set the ink, especially as the speed of the press increases. Thus efficient cooling of the unset ink, especially on both sides of the web in high-speed printing, remains a significant unsolved problem.
Another long-known problem is that of condensation of vaporized ink solvent and condensate streaking. When a hot printed web emerges from the dryer and approaches the first chill roll, residual ink solvent evaporates from the hot web surfaces. The vaporized solvent contacts and condenses on the relatively cold chill roll surface. The colder the roll the greater the condensation problem. As mentioned above, there is an inherent tendency for a film of air to be carried between the web and the cylindrical surface of the chill roll and be trapped therein thus preventing the desired intimate contact. The condensation can collect and build up in this air film space. If the web can wrap the chill roll with close intimate contact, the volume of this film of air can almost be eliminated. Consequently, the amount of condensate therein will be small and immediately reabsorbed by the web in such small amounts that the solvent will have no effect on the ink print because there is no room between the web and the chill roll surface for condensate to accumulate. If, however, the air film volume is larger, larger amounts of condensate can accumulate in this space and be reabsorbed in intermittent concentrated amounts sufficient to redissolve the ink and cause condensate streaking which ruins the printing.
While it is known that maximizing this intimate contact between web and chill roll will minimize condensate streaking, as discussed in U.S. Pat. No. 3,442,211, the prior art does not teach how this can be accomplished while also simultaneously maximizing the cooling and consequent setting of the ink on both sides of the web. The use of a metal pressure roll is not possible because of pressure distortion of the unset ink. The use of an elastomeric pressure roll has not been satisfactory because it has such a low index of contact temperature prevention that it does not efficiently cool the ink. Prior art teaches away from the use of water for directly cooling the web because it expands the fibers and causes loss of web tension. As a result of the above discussed problems, the prior art cooling apparatus and methods do not lower the temperature of the web and print as much as desired and require the use of a higher number of chill rolls than desired in the cooling zone of the press, and this increases both capital costs and servicing expenses. Even with these added chill rolls, uneven cooling across the web width exists and condensate streaking, ink picking and smearing remain unsolved problems.